This invention relates to an image forming apparatus such as a copying machine and a laser beam printer.
The sensitivity characteristics of a photosensitive member vary with time or variations of environmental conditions such as temperature and humidity. To make up for such variations, it has been proposed to measure the sensitivity characteristic of photosensitive medium by the provision of a sensor, control the surface status of a photosensitive drum by varying the corona charger grid voltage or exposure energy at the time of exposure to image through comparison with the measured characteristic and control the output image density to be constant by varying image forming conditions in accordance with detected temperature or humidity by further adding temperature and/or humidity sensor means to the image forming apparatus.
FIG. 36 shows a prior art example of setting the conditions for image forming through control of the corona charger grid voltage and setting optimum conditions through switching the amount of exposure to laser beam between two levels when a forming condition range exceeds a grid voltage range.
This method is for controlling the photosensitive drum surface potential contrast as image forming condition. In this case, under a constant exposure amount condition, the grid bias is varied continuously, thus varying surface potential V.sub.D on the photosensitive medium to vary the grid bias for obtaining desired potential contrast V.sub.D -V.sub.L. However, where the desired potential contrast range is broad, a sufficiently broad variation range can not be obtained with a single exposure amount. For this reason, it is proposed to permit switching of the exposure amount between levels for high and low potential contrast ranges as shown in FIG. 37. More specifically, when the potential contrast exceeds point B, the exposure amount is switched from level Lo to level Hi. In this case of exposure amount switching, the potential contrast control is performed such that the grid bias is varied with the exposure amount to obtain the same potential contrast B and B', as shown in FIG. 36. With this method, the potential contrast is controlled from A through B and B' to C in a case where it is varied in the increasing direction, while it is controlled from C through B' and B to A when it is varied in the reducing direction. In this way, it is possible to control the potential contrast over a wide range.
However, the sensitivity characteristic of photosensitive media varies with individual media. In addition, even with photosensitive medium having identical sensitivity charactetistic, there are liable to characteristic variations because of fluctuations of current characteristics of laser in exposure system and optical efficiency of optical system depending on the image forming apparatus.
With the prior art example noted above, although it is possible to make up for variations with time, it is impossible to make up for initial fluctuations of the sensitivity characteristic of photosensitive medium. For example, the relation between surface potential and grid bias voltage shown in FIG. 36 corresponds to either V.sub.D1 and V.sub.L1 or V.sub.D2 and V.sub.L2, as shown in FIG. 39, depending on the sensitivity characteristic of the photosensitive medium. If there is desired potential contrast D in this case, with a photosensitive medium having characteristics V.sub.D1 and V.sub.L1 the desired potential contrast can be obtained by setting grid bias voltage V.sub.G2, but with photosensitive medium with characteristics V.sub.D1 and V.sub.D2 it can not be obtained with initial preset light amount L.sub.0 (it being assumed that D&lt;B).
In such prior art example, even if the potential contrast is controlled to be equal at the time of switching of the exposure amount in a potential contrast range subject to the exposure amount switching, i.e., between B and B' in FIG. 36, the image quality, particularly the gradation, is subject to variations between points B and B' because of the influence of the V-E characteristic (i.e., relation between surface potential and exposure amount) accompanying the switching of the exposure amount.
More specifically, FIG. 38A shows V-E characteristic of a typical photosensitive medium such as OPC (organic photo-semiconductor). As is seen, in this V-E characteristic the potential is not varied linearly with the exposure amount. Actual V-E characteristic is as shown in FIG. 38B depending on the maximum exposure amount in use.
Therefore, in case if photosensitive medium control is carried out by the method noted above such as to obtain potential contrast at point B (B') in FIG. 36, there is a problem that the image quality of the output image is subject to variation when the exposure amount switching is effected with a change in the desired potential contrast due to a slight environmental variation. Heretofore, it has been in practice to effect correction of gradation at the time of such exposure amount switching. However, it has been difficult to stabilize the image quality with correction corresponding to the exposure amount change. Particularly, with a full-color image forming apparatus, in which importance is attached to the gradation, it is necessary to stabilize the gradation.